A four-wheel-drive or all-wheel-drive vehicle is one in which the engine powers all four wheels either part of the time or all of the time. Most cars have two-wheel drive, which means either the front or back wheels are powered by the engine.
Four-wheel or all-wheel drive, which we'll refer to as 4WD, is optional on many SUVs and there are pluses and minuses to this pricey and complex feature.
As the chart below illustrates, SUVs with 4WD drive are more expensive than their two-wheel-drive counterparts.
The added expense of 4WD varies by model because some systems are more complex or robust than others. Certain SUVs, like the BMW X3 and X5, Mercedes-Benz M-Class and Honda Pilot only come with 4WD, so deciding that you don't want 4WD will narrow your options.
Added traction is the biggest advantage to four-/all-wheel drive and is the reason these systems are now so popular. While the earliest four-wheel-drive systems were designed for traversing rough terrain with boulders, mud and steep landscape, modern, electronically controlled 4WD systems improve traction on asphalt and pavement. Extra traction provided by these systems can be especially handy on slippery roads (in rain, snow, dirt, gravel and mud), where it reduces the danger of losing control of the vehicle should one or all of the wheels lose grip.
Four-/all-wheel drive can also help when towing and hauling heavy loads. The added weight of a large payload or trailer can destabilize a vehicle and create problems with traction, which 4WD drive can work to correct.
The harsher the winters where you live, the more justifiable 4WD becomes. But the reality is that most drivers will do just fine with two-wheel drive, especially now that increasing numbers of vehicles have electronic traction control and stability control as standard or optional equipment.
Even in all but the harshest of snowy conditions, a two-wheel-drive vehicle (whether front- or rear-driven) will perform safely and securely in snow with the proper snow tires mounted on all four wheels. Comparison tests performed by automotive-enthusiast magazines in the snow have shown that a 2WD drive vehicle with snow tires on all four wheels will outperform a 4WD vehicle with regular tires.
All of this is to say that the added assurance of having all-wheel drive can be largely psychological. The instances in which most motorists who don't venture off-road actually benefit from having 4WD are relatively few, unless winters are harsh where they live.